An aliphatic polyester resin in which a polyhydroxy acid skeleton is a main component comprises a polymer of lactic acid or lactide (polylactic acid) or a copolymer of lactic acid or lactide with other monomer. Since such a resin has a polyhydroxy acid skeleton having an excellent biodegradability, studies for its wide practical use have been briskly carried out in recent years from the viewpoint of environmental problems.
The aliphatic polyester resin in which a polyhydroxy acid skeleton is a main component is usually produced by a process where a lactide which is a dimer of hydroxy acid is melted and mixed under heating together, if desired, with other hydroxy acid, polyol, lactone, etc. and is subjected to a thermal ring-opening polymerization under a nitrogen atmosphere using a known catalyst for ring-opening polymerization. This polymerization catalyst needs to have the activity during the polymerization reaction but, if it still remains in the resin in a state of having the activity even after completion of the polymerization reaction, it partially depolymerizes the resin by heating for removal of unreacted monomer and returns to the starting monomer lactide. The same thing also happens by heating for molding the manufactured resin or for conducting adhesion or coating process using the manufactured resin. When a monomer lactide is present in the resin, the lactide is degraded by moisture in the air to produce an organic acid and the organic acid cleaves the polymer chain whereby the strength of the resin is deteriorated. Accordingly, there has been a demand to lower the activity of the polymerization catalyst in the resin after completion of the polymerization reaction so that production of a monomer lactide by heating after the polymerization or after the manufacture is prevented.
As to a method therefor, there have been proposals where a specific phosphorus compound is added to the reaction system after completion of the polymerization reaction to lower the activity of the polymerization catalyst (Patent Documents 1-3). With regard to the phosphorus compound as such, a phosphoric acid compound or a phosphorous acid compound is used in the Patent Documents 1 and 2 while alkyl phosphate and/or alkyl phosphonate are/is used in the Patent Document 3.
According to the methods of Patent Documents 1 to 3, activity of the polymerization catalyst remained in the resin after completion of the polymerization can be lowered to some extent and production of a monomer lactide by heating after the polymerization or after the manufacture can be prevented to some extent. However, the degree of prevention is insufficient for fully preventing the deterioration of strength of the resin whereby there has been a demand for further improvements.